In the book Hitler's Scientists: Science, War, and the Devil's Pact by John Cornwell there is a chapter devoted to Radar whereby the author claims that the first experiments using radio location were done in the mid-1930s. He cites Robert Page of the U.S. constructing a radar device in December 1934. Robert Watson-Watt is claimed to have pioneered radar in February 1935. Cornwell gives credit to Germany constructing radar in March 1935. If Cornwall had done his homework, he would have consulted David Pritchard's book The Radar War: Germany's Pioneering Achievement 1904-45.

Pritchard is the first author, I believe, to give credit for the invention of radar to Christian Hülsmeyer whom he gives the title of The Father of Radar. Hülsmeyer built a primitive radar system in 1904 in Düsseldorf which was reported at the time in the 18 May 1904 edition of the Kölnischer Zeitung. He approached the German Navy for financial
assistance but Admiral von Tirpitz was not interested. He received a patent for his discovery on 30 April 1904 in Germany. He also received a patent in England.

Since Germany lost all her rights to any patents prior to and during WW I by the Versailles Treaty, it can be credibly stated that Germany had no patent to radar and that the patent granted to Watson-Watts in 1935 makes him the "inventor" of radar as stated in mosthistory books.

Did Watson-Watts use information in the 1904 patent to construct his own radar device? This would easily have been available to him.

The US Navy was detecting aircraft with radio waves in the late 1920s. As this effort was accomplished by a US Government agency, I am not certain that any effort was made to patent the device. However, this work basically continued unbroken and there is a German paper in 1936 refering to Doppler radar research being performed by GE in the USA. For the Germans to have known about it, it seems probable that it had been progressing for some time prior to their becoming aware of it. I believe that the USA work was entirely independent of anything happening in GB or Europe during that period. It was not untill later that GB and USA pooled their efforts in this field. I believe that most USN capital ships were equipped with radar for detection and gun laying well before WWII.

You may also want to locate the German language books by Fritz Trenkle on this subject.

In English, there is an excellent book entitled "GEMA: Birthplace of German Radar and Sonar" by Harry von Kroge. You can view it at amazon.com The back cover writing is especially enlightening, and the contents list can be seen as well.

The way the story is usually told, Watson-Watt was actually asked to investigate the possibilty of using radio waves to damage or destroy aircraft (was a "death ray" feasible, in fact) and in responding to that enquiry from the government, it came up that radio waves were disturbed by aircraft. After that, the issue of detection was followed up.

IIRC from pritchard's book, Hülsmeyer's device was intended to give a proximity alert to ships in fog by sounding a klaxon. It wasn't, in 1904–05 giving ranges and bearings. Quite an achievement nonetheless.

I think that the cathode ray tube was invented around the same time, so sooner or later someone was going to bring the two ideas together, I guess.